1. Field of the Invention
The invention relates to the field of disk drives and more particularly to methods for writing servo tracks on magnetic hard disks.
2. Description of Related Art
Hard disk drives provide prerecorded tracking servo information on the data recording surfaces of their magnetic hard disks. This servo information typically comprises servo bursts spaced evenly along tracks. Data is recorded between the servo bursts. In most cases, servo bursts are approximately radially aligned, describing a small arc from the disk's ID to its OD. This radial alignment makes them look like arced spokes of the wheel. They are made to form and arc because the servo data is read by a rotary actuator that describes the same arc as it the traverses between a disk's ID and its OD.
FIG. 1 illustrates a disk 10 having a number of servo data spokes 12. While there are eight illustrated in the figure, a disk drive will typically have hundreds of such servo data spokes spaced it even angles around disk. In many disk drives today, the servo data takes up approximately 3 to 11 percent of the total disk drive recording surface.
The servo bursts may be written onto a disk's surface using a variety of techniques. The most common method is to write the servo onto the disk using the disk drive's own magnetic head controlled typically by an externally introduced picker that grasps the drive's rotary actuator arm upon which the read/write head is mounted. An external mechanism incrementally moves the arm while other circuits command the disk drive to write the servo bursts.
Another common servo-writing method comprises writing servo bursts onto the disks already assembled onto the disk drive spindle but prior to the disk drive spindle/disk combination, also known as a hub/disk assembly (“HDA”), being assembled into the disk drive itself.
A newer approach employs a stamper to “print” the servo patterns on the disk using a high permeability stamper, as illustrated in FIG. 2, to impose a pattern on media. As illustrated in the top leftmost portion of the figure, the disk is first DC erased. For example, an externally applied field 13, the large arrows H in the figure, causes all the magnetic domains 14 of the media to switch in an uniform direction as illustrated. Next, a high permeability stamper 16, having a desired pattern composed of raised bosses 17, is pressed against the disk 10. An externally applied field of opposite polarity 18 illustrated by the now downward arrows 18, is now applied to the disk through the stamper 16. This causes the disk areas in contact with the stamper 16 to switch their magnetic direction to be aligned with the externally applied field. The areas not in contact with the stamper are shielded by the stamper. The shielded areas do not change their magnetic orientation. This causes the disk to assume a reversed magnetic orientation 15 in the pattern of the bosses 17 of the stamper 16.
While FIG. 2 illustrates vertically oriented magnetic domains 14 and 15 which are useful in perpendicular recording, the same technique may be employed using horizontally oriented magnetic fields to encode horizontally oriented magnetic domains.
The stamper 16 appears identical to FIG. 1 when viewed from a plan view. In other words, the stamper 16 would encode the images of the servo bursts radially aligned in arced spokes as illustrated in FIG. 1.
A problem that occurs in writing servo onto a disk regardless of the technique used is that the disk drives can tolerate only so much servo error before the servo must be rewritten or the disk scrapped. Most drives cannot, for example, tolerate two bad servo bursts in a row.
Today's disk drive manufacturing processes, therefore, typically check the quality of servo data recording immediately after data is written. This is preferably done after the disks are mounted in the production disk drive. However, the quality of the servo data recording may also be tested by spinstand using special test equipment.
When a stamper is used to print servo data onto a disk, the disk is not yet assembled onto the disk drives own spindle. If a servo rewrite becomes necessary, the disk must be remounted on the stamper apparatus. For this reason, disks made using stampers are typically tested on a spinstand.
It remains desirable however to both test the quality of servo data in the disk drive itself and to avoid rewriting the servo data if there are too many errors.
Another problem in the manufacture of magnetic disks is that different disk drives require different servo data patterns. When stampers are used to print the servo data onto the hard disk, a different stamper is required for each different kind of servo data pattern. This is undesirable from a cost perspective as stampers can be very expensive.